This invention relates to a circuit for protecting telephone lines from hazardous voltages. More particularly, it relates to a digital circuit which is substantially frequency independent and which protects telephone lines from certain hazardous voltages.
The difficulties caused by hazardous voltages, such as lightning and other voltages such as AC line voltages, on telephone lines has long been a problem in the telephone industry. Exposure of delicate equipment to these voltages, particularly solid state circuits, can ruin this equipment, and, even more importantly, can expose users of this equipment to these dangerous high voltages. The problems of very high voltage exposures, such as lightning, have been solved substantially in the use of lightning arresters connected directly between the telephone lines and ground. A lightning arrester normally consists of a high voltage breakover gas tube, commonly called a network protection device, such as the one disclosed in U.S. Pat. No. 3,725,613, assigned to the Rochester Telephone Corporation. However, the use of these voltage gas tubes will not provide protection for lower voltages, such as 120 volt AC, which can come on the telephone line due to, for example, a power line coming in contact with the telephone line. While this type of situation occurs quite often, it has not been adequately dealt with.
It has been suggested that one could use a threshold detector across the tip and ring of telephone line and further connected to a phase lock loop circuit in order to detect a specific frequency on the line and then open or close the telephone line circuit in response to the specific frequency as detected by the phase lock loop circuit. However, it has been found that this approach has many drawbacks.
One problem is that a phase lock loop circuit is programmed to a specific frequency, thus hazardous voltages on the line of different frequencies would not even be detected by this circuit, thus exposing the customer to these hazardous voltages. Furthermore, this phase lock loop circuit must be pre-programmed by a skilled technician and tuned to a specific frequency in order to be operable at all. Since hazardous voltages occur at various frequencies, this device has found little acceptance.
Similar circuits have been provided to isolate the central office from customer equipment for purposes of testing faults in the line. Examples of such circuits are shown in U.S. pat. No. 3,766,336, issued to Wikholm; U.S. Pat. No. 3,636,280, assigned to General Telephone Company of California; and U.S. Pat. No. 3,773,986, assigned to Communication Systems Corporation. However, none of these patents sense hazardous voltages on the line, but are only responsive to a specific signal sent from the central office in order to open the line so that the central office can test its line to see if a particular fault is customer related or telephone-operating company related.